Bug 1524396 - Unify how target/host linker/flags are passed to rust. r=chmanchester

The current setup uses different ways for different platforms, with
different workarounds, even using extra configuration items for Windows.

Now that there can't be a difference between the host per the build
system and the host per rust, we can get rid of those configuration
items, and use a more common infrastructure.

We cannot, however, avoid using wrapper scripts, because per-target rust
link-arg flags don't work up great.

The downside is that multiplies the number of wrappers, as we now have
to have a different one for host and target, and then we have .bat files
and shell scripts for, respectively, Windows hosts, and other hosts.

Depends on D24321

Differential Revision: https://phabricator.services.mozilla.com/D24322

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extra : moz-landing-system : lando